To provide optical power monitoring for light transmitting from VCSEL to fiber, additional optical elements are employed to divert portion of light power before being coupled to a fiber. The diverted light is then followed by light detection. Among different detection schemes, photodetector is the most commonly used because it gives electrical output that can feedback to electronic circuit directly and easily. However, various schemes had been developed for light extraction in different optical designs. Few main schemes, such as diffraction grating, micro-structure on optical surface and beam-splitting by partial reflective filter, had been proposed for light extraction purposes.
FIG. 2a (U.S. Pat. No. 6,636,540) shows key elements of the coupling system include a collimation lens for light emitting from VCSEL, a beam splitter to divert portion of light from optical path to output fiber, and a focusing lens to couple light into the output fiber. Owing to dimension constraint within an optical transceiver, this coupling lens is just few centimeters in size and requires dimension accuracy, especially on optical surfaces. The industrial manufacturing solution for such optical system is by precise injection molding with high-temperature thermoplastic, such as Polyetherimide (e.g. Ultem). The refractive index of Ultem is about 1.62 and has lower material absorption at the VCSEL emission wavelength around 850 nm. Collimated light beam incident on the 45 degrees slanted interface formed with lens body and air, undergone total internal reflection (TIR) and light beam direction was bent by 90 degrees.
In U.S. Pat. No. 6,636,540, a concave indention was included in lens body where 90 degrees turned collimated optical beam intercepts. A two-part optics, namely a beam splitting plate and a wedge, where the reflection at the interface was used to provide feedback. Given the lens size mentioned earlier, assembly process of two miniature size optical parts into the desired position in the indention on the lens is rather difficult to keep variation in optical alignment small.
A modified optical design by combining the beam splitting plate and the wedge into a single piece, the prism, was disclosed in FIG. 2b (U.S. Pat. No. 8,787,714). The assembly process was simplified but the optical alignment to output fiber was still sensitive to the position of the prism in filler matrix.
FIG. 2c (U.S. Pat. No. 9,116,312) and FIG. 2d (U.S. Pat. No. 9,166,694) depict another two schemes in which shape matched beam splitting insert was assembled into a lens cavity. Comparing to FIG. 2a, the advantage of this approach is that it helps to reduce optical alignment variation, in turns smaller fiber coupling efficiency variation caused by assembly process can be obtained.
The above description of the background is provided to aid in understanding an optical coupling subassembly, but is not admitted to describe or constitute pertinent prior art to the optical coupling subassembly, or consider the cited documents as material to the patentability of the claims of the present application.